- Sudoku X adds two units: the main diagonal (R1C1–R9C9) and anti-diagonal (R1C9–R9C1)
- Both diagonals must contain each digit 1–9 exactly once
- The center cell R5C5 belongs to both diagonals, its row, its column, and its box — 5 units total
- The 4 corner cells each belong to one diagonal — giving them an extra constraint
- Treat each diagonal exactly like a row or column during scanning
The Two Diagonals as Extra Units
In Sudoku X, the two main diagonals are treated as full units — like additional rows or columns. The main diagonal runs from the top-left corner (R1C1) to the bottom-right corner (R9C9). The anti-diagonal runs from the top-right (R1C9) to the bottom-left (R9C1).
Each diagonal contains exactly 9 cells. Each must hold every digit from 1 to 9 exactly once. This brings the total number of units to 29 (9 rows + 9 columns + 9 boxes + 2 diagonals).
Most Constrained Cells
| Cell | Units It Belongs To |
|---|---|
| R5C5 (center) | Row 5, Column 5, Center Box, Main Diagonal, Anti-Diagonal |
| R1C1, R9C9 | Row, Column, Corner Box, Main Diagonal |
| R1C9, R9C1 | Row, Column, Corner Box, Anti-Diagonal |
| R1C5, R5C1, etc. | Standard 3 units only (not on either diagonal) |
Using Diagonal Constraints in Solving
Treat each diagonal exactly like a row or column: cross-hatch each digit across the diagonal just as you would scan a row. When a digit appears elsewhere in a diagonal, eliminate it from other diagonal cells. Locked candidates apply to diagonals too: if a digit's candidates in a diagonal all lie within one box, eliminate that digit from the rest of that box.
Diagonal Interaction Patterns
The most useful interaction: the diagonal intersects 5 of the 9 boxes (for the main diagonal: top-left, one each of the second-row boxes, center, and bottom-right). A digit's placement in any of these boxes constrains the diagonal, and vice versa. Working these cross-unit interactions is the primary technique that distinguishes Sudoku X solving from standard solving.
Sudoku X puzzles are available in the variants section. Try one after completing a standard Hard puzzle — the extra constraints usually make Sudoku X of equivalent rating slightly easier than standard Sudoku.